This invention relates to an absolute-type encoder for generating position information by being mounted on a drive shaft of a rotationally driven element such as a servomotor or the like, or on a linearly driven element such as a linear actuator or the like.
Encoders serve as devices for detecting rotational and linear positions of machines such as servomotors, linear actuators, tachometers, and the like, to allow accurate positioning of such machines, and determination of such quantities as velocity and acceleration. Many different kinds of encoders are available for such purposes.
Incremental-type encoders are typically used for positioning servomechanisms due to their simplicity and low cost. However, incremental-type encoders require initializing whenever power is removed from the device. Initializing the encoder requires sensing a "home" position and then using the home position as a reference for subsequent position measurements. If the initializing process includes an error, then all subsequent measurements will include the error.
Absolute encoders also require initializing to a home or reference position upon being powered up for the first time. However, in contrast to incremental encoders, absolute encoders do not need to be re-initialized every time the power is turned on. Thus, after a one-time adjustment, an absolute encoder will provide position information immediately upon start up, without the need for re-initialization.
Typical absolute encoders are of the so-called discrete or digital type wherein position information is encoded on a rotary disk or a linear transducer element as binary ones and zeroes, or as on and off states. The binary information is typically encoded as a series of concentric rings or bands on a rotary disk, or as a series of adjacent bands or strips on a linear encoder. Typically, one ring or strip, corresponding to the most significant binary digit, is divided into two equal parts, with one part representing a one, and the other part representing a zero. Successive adjacent rings or strips are divided into twice as many equal alternating ones and zeroes as the previous rings or strips. The final ring or strip, having the greatest number of equal-sized ones and zeroes, corresponds to the least significant binary digit. Each distinct binary number can be associated with a unique rotational or linear position.
The ones and zeroes may be represented as alternately light and dark optically reflective domains, or as magnetic fields of alternating polarity, or as discrete variations in height or width wherein one height or width measurement corresponds to a one, and a second different height or width measurement corresponds to a zero.
Because of the two-valued nature of binary or digital encoders, intermediate values are not permitted as they give rise to ambiguous or indeterminate values, and hence, loss of precision in rendering position measurements. Thus, typical digital or binary encoders require sharp transitions between alternating regions, and relatively constant values within regions.
Digital or binary encoders are inherently limited in their fundamental accuracy by the number of rings or strips they possess, as well as by the size of the smallest regions in the least significant ring or strip. Within one of such regions, a rotary disk or linear actuator may be positioned anywhere, yet yield the same digital value, so that the potential measurement error is approximately equal to the size of the smallest binary region within the least significant ring or strip. Also, in order to achieve ever finer resolution, additional rings or strips must be added, thereby consuming more space, requiring additional sensors for each ring or strip, and adding to encoder complexity.